The present invention relates generally to magnetic circuit components, such as transformers and inductors. More particularly, the present invention relates to devices and methods for reducing leakage inductance in magnetic components.
Power converters are used in a variety of applications in electronic devices such as lighting ballasts and drivers for high voltage lamps. Historically, conventional high voltage power converters can include an isolated transformer. In some applications such as flyback transformers and traditional or modified buck-boost type power converters, the isolated transformer can act as a bridge between primary and secondary circuits. In some applications, the primary circuit includes a voltage source and can be referred to as an input circuit, and the secondary circuit includes a device to be powered and can be referred to as an output circuit. The secondary circuit can also be coupled to a device to be powered by the power converter. Conventional transformers of this type can be used in high voltage applications where the transformer acts as a step-up or step-down transformer and can include a rectifier or an inverter. In some conventional applications, transformers of this type are used for increasing an input voltage to a much higher output voltage. For example, conventional plasma lamp power supplies and high voltage ballasts for other types of conventional lighting driver circuits include isolated transformers.
One problem associated with conventional power converters that utilize isolated transformers is leakage inductance. Leakage inductance can occur when the windings in the primary and secondary transformer coils are either improperly positioned, improperly insulated, or make improper contact. Other defects in one or more windings, in the bobbin structure, or in the inter-layer insulation can also cause leakage inductance. Conventional transformers known in the art are particularly susceptible to leakage inductance because of their winding configurations. The effects of leakage inductance can include reduced magnetic flux between primary and secondary coils and inefficient power regulation in high voltage applications. Leakage inductance also causes power loss and can reduce transformer efficiency.
Because an isolated transformer is generally formed between the input and output circuits in some conventional power supplies, managing leakage inductance is important for maximizing power conversion efficiency and for providing proper power regulation to the output circuit. For example, if the leakage inductance is too high in a flyback converter, switching transitions are slowed down, energy is lost, and in some applications a high voltage ring can occur when the main switch is turned off, causing a large voltage stress on the switch and an undesirable power loss. Such stress can cause a switch to fail or can permanently damage other circuit components.
Others have attempted to solve the problems associated with leakage inductance in high voltage power converters, switching power supplies, and specifically in flyback converters and flyback transformers, by splitting the primary and secondary windings into discrete insulated layers and interleaving the layers. The conventional layer interleaving technique mitigates leakage inductance in some applications. However, in other applications, especially where the number of primary turns is greater than the number of secondary turns, or vice versa, conventional interleaving configurations become impractical and do not adequately control leakage inductance.